function [ya,yp] = HPA(xa,xp)
%DHPA Summary of this function goes here
%   Detailed explanation goes here

%
% ya=xa;
% yp=xp;
if xa>1
    ya=1;
else
    ya=(2*xa)./((xa).^2+1);
end
yp=(pi/3)*((xa.^2)./(xa.^2+1))+xp;
